In the production of hydrocarbons such as gas and crude oil from subsea wells, similarly to land based wells, fluid is normally urged to the surface by gas pressure within the subterranean formation. As the multiphase, liquid gaseous flow is received at the surface of the water, it is separated into discrete components. If the primary flow is in the form of liquid, the gas is often flared off or otherwise disposed of. When the gaseous element constitutes a considerable proportion of the overall flow, it can be treated and further used commercially.
In the instance of some subterranean formations, the hydrocarbon liquid can only be recovered or produced with the aid of some form of reservoir enhancement. At some locations for example, production can be fostered by the pressurized injection of water into the formation. Such an injection or flooding process urges the oil through the reservoir and toward one or more producing wells where a composite stream of water, oil, and gas can be readily produced.
A further facility for raising crude oil to the surface under reduced low pressure conditions is by a gas assist or gas lift procedure. In this method, gas is intermixed with the crude oil in an amount, and in such a manner, to decrease the viscosity and specific gravity of the liquid. The latter can then be more readily raised from a subterranean reservoir to the water's surface.
In either instance, the composition of the produced crude product will normally embody a mixture or emulsion of cruse oil, gas, and water. As this emulsion is brought to the surface it is treated to permit individual streams of the water, oil and gas to be either further used, transported or otherwise disposed of for commercial purposes.
Where the composite or integrated hydrocarbon stream is transmitted from an underwater well to a remotely positioned facility, there will be a propensity for the various elements to separate out. More specifically, as the composite stream is pushed or urged across the ocean floor, there will be a considerable amount of heat transfer between the stream and the surrounding environment. Where the water constitutes a relatively cold environment, a substantial amount of heat will be lost through the conductor walls.
When the product transmission takes place over a relatively long distance, particularly along the sea floor, separation of the stream into discrete components will form discrete slugs of liquid and gas which are moved toward the processing equipment. It can be appreciated that eventually the crude oil and the gas will arrive at the processing facility in a condition where the liquid must be pumped from the ocean floor to separating and treating equipment on a marine structure or onshore.
It can be further appreciated that under these circumstances where a pumping facility is utilized to lift the liquid component, introduction of a substantial amount of gas into the pump inlet will result in erratic operation of the pump. The normal consequence is a sporadic flow of hydrocarbon to the surface positioned processing equipment.
Toward overcoming these inherent problems in the production of oil and gas from an offshore location, there is presently provided means for producing and transmitting a composite or integrated flow of gas, oil and water across a relatively long distance prior to the stream being lifted to a surface positioned treating and storing facility. The composite flow is initially directed through a pipeline along the ocean floor. At the terminal point such as a marine structure, platform or the like, the composite stream is delivered to a multiphase separator. In the latter, the stream is segregated into discrete liquid and gaseous segments.
The gaseous segment is delivered to the deck of the platform apart from the liquid flow. In the instance of the latter, the water-oil emulsion is first accumulated in a subterranean sump or reservoir prior to being removed at a controlled rate.
It is therefore an object of the invention to provide a subsea production and product transmission system which is capable of conducting a composite, multiphase stream of produced product to a fluid separator and thence to fluid processing equipment.
A further object is to provide a subsea system which includes a plurality of satellite wells which are remote from, and communicated with processing equipment.
Another object is to expand the use of a fixed, offshore structure to accommodate an increased volume of hydrocarbons for processing, without adding substantially to the structure's weight.
A still further object is to provide a self-supporting, fluid conducting riser for use on an offshore fluid processing installation.